Liquid absorbing sheet and nonaqueous electrolyte battery pack

ABSTRACT

A liquid absorbing sheet has a liquid-absorbing resin layer that can effectively absorb the nonaqueous electrolyte solution used in nonaqueous electrolyte secondary cells that make nonaqueous electrolyte battery packs (in particular, lithium ion-based nonaqueous secondary battery packs). The liquid-absorbing resin layer is obtained by irradiating a particular monomer composition with UV rays or other energy rays to polymerize the monomer composition. The monomer composition contains a monofunctional monomer component (A) having a polyethylene glycol acrylate monomer and an amide bond-containing acrylic monomer; and a polyfunctional monomer component (B).

TECHNICAL FIELD

The present invention relates to a liquid absorbing sheet for absorbingan electrolyte solution when such a solution leaks from a nonaqueouselectrolyte battery cell encased in a nonaqueous electrolyte batterypack. The present invention also relates to a nonaqueous electrolytebattery pack that uses such a liquid absorbing sheet.

BACKGROUND ART

Battery packs are widely used that contains a plurality of primary orsecondary battery cells, a circuit board, and a battery case encasingthese components. When an electrolyte solution leaks from any of thebattery cells, it can corrode the wiring of the circuit board, wherebythere may occurs a conduction failure or short circuit. To prevent suchcorrosion and short circuits when leakage of the electrolyte solutionoccurs, a liquid-absorbing element containing an liquid absorbing agentcapable of absorbing the electrolyte solution is arranged in the batterypack adjacent to or in the vicinity of the battery cell (Japanese PatentApplication Laid-Open No. 2001-351588). Various polymer materials areused as the liquid absorbing agent, including adsorbents, gellingagents, and self-swelling agents. Among specific examples of the liquidabsorbing agents described are polyacrylate-based water-absorbingresins, starch/graft copolymer-based water-absorbing resins, polyvinylalcohol-based water-absorbing resins, polyacrylamide-basedwater-absorbing resins, isobutyrene-maleic acid copolymer-basedwater-absorbing resins, long chain alkyl acrylate crosslinked polymers,and polynorbornens.

One drawback of these liquid absorbing agents is that they cannoteffectively absorb propylene carbonate, dimethyl carbonate and othercarbonate-based solvents that are widely used in nonaqueous electrolytebattery packs, a type of batteries that have become increasingly used inrecent years. Specifically, these solvents are used in nonaqueouselectrolyte secondary cells that make lithium ion-based nonaqueouselectrolyte secondary battery packs. Another drawback is that theseadsorbents show little or no adhesion, so that they cannot be easilystuck to a nonwoven fabric support at room temperature and thus, requirea thermal laminator. Furthermore, an adhesive layer must be arranged toapply the liquid absorbing agent to nonaqueous electrolyte secondarybattery packs. As a result, the usable amount of the adhesive isdecreased by an amount corresponding to the thickness of the adhesivelayer.

The present invention addresses the above-described problems and to thatend, it is an object of the present invention to provide a liquidabsorbing sheet that shows adhesion and is capable of effectivelyabsorbing the nonaqueous electrolyte solution used in nonaqueouselectrolyte secondary cells that make nonaqueous electrolyte batterypacks (in particular, lithium ion-based nonaqueous secondary batterypacks). It is another object of the present invention to provide abattery pack equipped with an electrolyte-absorbing element made of sucha liquid absorbing sheet.

DISCLOSURE OF THE INVENTION

In the course of our study, the present inventors have found that aresin layer obtained in a particular manner can absorb and retainsignificant amounts of an electrolyte solution and shows adhesion, thefinding leading to the present invention. Specifically, this resin layeris obtained by irradiating a particular monomer composition with anenergy ray to cause it to polymerize. This monomer composition containsa monofunctional monomer component (A) comprising a polyethylene glycolacrylate monomer and an amide bond-containing acrylic monomer; and apolyfunctional monomer component (B).

Specifically, the present invention provides a liquid absorbing sheethaving a liquid-absorbing resin layer, wherein the liquid-absorbingresin layer is obtained by irradiating a monomer composition with anenergy ray to polymerize the monomer composition, the monomercomposition containing a monofunctional monomer component (A) having apolyethylene glycol acrylate monomer and an amide bond-containingacrylic monomer; and a polyfunctional monomer component (B).

The present invention also provides a nonaqueous electrolyte batterypack having a nonaqueous electrolyte battery cell, a circuit board, anelectrolyte-absorbing element for absorbing an electrolyte solution inthe event of electrolyte leakage from the nonaqueous electrolyte batterycell and a battery case encasing all of the above components, whereinthe electrolyte-absorbing element is formed of the above-describedliquid absorbing sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are each a cross-sectional view of a liquid absorbingsheet of the present invention, and FIG. 1C is a perspective view of theliquid absorbing sheet of the present invention.

FIG. 2 is a perspective view showing a nonaqueous electrolyte batterypack of the present invention.

FIG. 3 is a perspective view showing another nonaqueous electrolytebattery pack of the present invention.

FIG. 4 is a diagram illustrating an electrolyte absorption testconducted using a model battery pack.

BEST MODE FOR CARRYING OUT THE INVENTION

First, a liquid absorbing sheet of the present invention will bedescribed as below.

In one embodiment, the liquid absorbing sheet of the present inventioncan be provided as an independent sheet formed entirely of a liquidabsorbing resin layer 1 as shown in FIG. 1A. In another embodiment, theliquid absorbing sheet may consist of a substrate 2 and theliquid-absorbing resin layer 1 disposed on one side of the substrate 2as shown in FIG. 1B. In the liquid absorbing sheet of the presentinvention, the liquid-absorbing resin layer 1 can itself absorb andretain significant amounts of an electrolyte solution and itself hasadhesion, so that the need to provide a separate adhesive layer iseliminated or even when such an adhesive layer is used, the thickness ofthe adhesive layer can be made thin. As a result, the proportion byvolume of the liquid-absorbing resin layer 1 in the liquid absorbingsheet can be made increased. This facilitates the placement of theliquid absorbing sheet in the battery case.

Although the substrate 2 for use in the liquid absorbing sheet of thepresent invention may be a resin film that is impermeable to electrolytesolutions (such as a plastic film made of polypropylene or othermaterials), it may be a material that can absorb and retain thenonaqueous solvent, including nonwoven fabric or synthetic paper formedof plastic fibers such as polypropylene, or other materials. Thesubstrate made of such a nonwoven fabric can absorb the nonaqueoussolvent at an increased rate.

The liquid-absorbing resin layer 1 to form the liquid absorbing sheet ofthe present invention comprises a polymer film obtained by irradiating aparticular monomer composition with an energy ray such as ultravioletray and electron beam to polymerize the composition, thereby making asheet. According to the present invention, this monomer compositioncomprises a monofunctional monomer and a polyfunctional monomer to serveas a crosslinker.

According to the present invention, the monofunctional monomer component(A) comprises a polyethylene glycol acrylate monomer and an amidebond-containing acrylic monomer. The polyethylene glycol acrylatemonomer and the amide bond-containing acrylic monomer each form anon-adhesive homopolymer, but, together, they unexpectedly form anadhesive copolymer.

Such a polyethylene glycol acrylate monomer may be a phenoxypolyethyleneglycol acrylate with an ethylene oxide-added mol number of preferably 1to 30, more preferably 6, or it may be a methoxypolyethylene glycolacrylate with an ethylene oxide-added mol number of preferably 1 to 30,more preferably 3 or 9.

Preferably, the amide bond-containing acrylic monomer isacryloylmorpholine or N,N-diethylacrylamide.

The molar ratio of the polyethylene glycol acrylate monomer to the amidebond-containing acrylic monomer to compose the monofunctional monomercomponent (A) is adjusted to a suitable range, since the adhesion of theliquid absorbing sheet will be decreased if the amount of the formerrelative to the latter is too much or too less. The amount of the amidebond-containing acrylic monomer is preferably in the range of 20 to 70parts by weight, more preferably in the range of 40 to 60 parts byweight with respect to 100 parts by weight of the polyethylene glycolacrylate monomer.

According to the present invention, the above-described polyethyleneglycol acrylate monomer and the amide bond-containing acrylic monomermust be present in the monofunctional monomer component (A) preferablyin a total amount of at least 20 mol %, since too small an amount ofthese components tends to result in a decreased absorption of thenonaqueous solvent.

As long as the advantages of the present invention are not affected, themonofunctional monomer component (A) may contain, aside from thepolyethylene glycol acrylate monomer and the amide bond-containingacrylic monomer, other monofunctional monomers, such ashydroxyethylacrylate, acrylic acid, 2-ethylhexylacrylate, and laurylacrylate.

According to the present invention, the polyfunctional monomer component(B) serves to introduce crosslinks into the liquid absorbing resin layer1 and is preferably a monomer having 2 or more acrylate residues.Examples thereof include hydroxypivalic acid neopentyl glycoldiacrylate, polyethylene glycol diacrylate (ethylene glycol-added molnumber (n)=14), bisphenol A diacrylate, phenyl glycidyl ether acrylate,and hexamethylene diisocyanate prepolymer.

The polyfunctional monomer component (B) is added to the monomercomposition in an amount to give a crosslink density of preferably from0.0001 to 0.17, more preferably from 0.001 to 0.1. Too small an amountof the polyfunctional monomer component (B) may result in a decreasedability of the liquid absorbing resin layer 1 to retain its shape,whereas too large an amount may lead to a decreased ability of theliquid absorbing resin layer 1 to absorb the nonaqueous solvent.

Given that ‘a’ indicates the number of the functional groups borne by asingle molecule of the polyfunctional monomer, ‘b’ indicates the numberof mols of the polyfunctional monomer present in the monomercomposition, and ‘c’ indicates the number of mols of the monofunctionalmonomer present in the monomer composition, the crosslink density can bedefined by the following equation:Crosslink density=a×b/(b+c).

The first construction of the liquid absorbing sheet of the presentinvention as depicted in FIG. 1A can be obtained by coating a peelablefilm, such as polyethylene terephthalate film, with the above-describedmonomer composition containing the monofunctional monomer component (A)and the polyfunctional monomer component (B), irradiating the coatedfilm with an energy ray to polymerize the composition and thereby form asheet, and peeling the sheet off the peelable film. The secondconstruction as depicted in FIG. 1B can be obtained either by coating anonwoven fabric with the monomer composition and polymerizing thecomposition, or by laminating a nonwoven fabric onto the firstconstruction of FIG. 1A. The monomer composition can be applied to thepeelable sheet or nonwoven fabric by any suitable conventionaltechnique, such as roll coater technique. The polymerization using anenergy ray is typically carried out at 15 to 25° C. under theirradiating of a UV ray with a wavelength of 250 to 350 nm at an energydensity of 100 to 2000 mJ/cm². When it is desired to attach a nonwovenfabric to the liquid absorbing sheet comprising the single-layered,liquid absorbing resin layer 1 as shown in FIG. 1A, the nonwoven fabriccan be easily attached to the liquid absorbing sheet 1, which itselfshows adhesion, at room temperature with the help of simple equipmentsuch as hand roller. Considering the fact that the leakage in many casesoccurs at the cathode of the cylindrical batteries, the liquid absorbingsheet is preferably shaped as a doughnut-like shape as shown in its planview in FIG. 1C so that it can be applied about the cathode terminal.

If necessary, a flame retardant (e.g., aluminum hydroxide and melaminecyanurate) may be further added to the liquid-absorbing resin layer 1 ofthe liquid absorbing sheet of the present invention. This imparts aflame retardancy to the liquid absorbing sheet.

The liquid absorbing sheet of the present invention is suitable as anelectrolyte-absorbing element used in a nonaqueous electrolyte batterypack that consists of a battery case encasing nonaqueous electrolytebattery cells, a circuit board and the electrolyte-absorbing element.The liquid absorbing sheet serves to absorb the electrolyte solutionshould leakage occur from the battery cell. One example of such abattery pack is shown in FIG. 2. The battery pack includes a batterycase 21, which encases a circuit board 22 and a plurality of nonaqueouselectrolyte battery cells 23 arranged on the circuit board 22. A liquidabsorbing sheet 26 as described above with reference to FIG. 1A isarranged between the circuit board 22 and the nonaqueous electrolytebattery cells 23 for absorbing the electrolyte solution should leakageoccur from any of the nonaqueous electrolyte battery cells. A metal lead24 connects the circuit board 22 with each of the nonaqueous electrolytebattery cells 23. The metal lead 24 is also connected to externalterminals 25. As shown in FIG. 3, a liquid absorbing sheet 27 asdescribed above with reference to FIG. 1B may be arranged on top of thenonaqueous electrolyte battery cells 23 with its substrate facing thenonaqueous electrolyte battery cells 23.

Although the battery cases of the respective nonaqueous electrolytebattery packs shown in FIGS. 2 and 3 are rectangular parallelepiped withcylindrical battery cells, the shape and arrangement of the battery caseand the battery cells, as well as the type of the battery cells, are notlimited to those shown in the figures and may vary depending on theintended purposes of the batteries.

As set forth, the nonaqueous electrolyte battery packs of the presentinvention use the liquid absorbing sheet as an element for absorbing thenonaqueous electrolyte solution, where the liquid absorbing sheetcomprises the liquid-absorbing resin layer that is obtained byirradiating a monomer composition with UV or other energy rays, thecomposition contains a monofunctional monomer component (A), whichcomprises a polyethylene glycol acrylate monomer and an amidebond-containing acrylic monomer, and a polyfunctional monomer component(B). The nonaqueous electrolyte battery packs of the present inventioncan accordingly offer high absorbability and retainability of thenonaqueous electrolyte solution and significantly reduces the occurrenceof corrosion and short circuits of the circuit board even in the eventof leakage of the nonaqueous electrolyte solution from the batterycells. The liquid-absorbing resin layer, which itself shows adhesion,makes it possible to easily attach the liquid-absorbing resin layer to asubstrate or easily attach the liquid absorbing sheet to the batterypack, at room temperature and without relying on thermal lamination.

EXAMPLES

The present invention will now be described in detail with reference toexamples.

Examples 1 Through 6 and Comparative Examples 1 Through 7

A monofunctional monomer(s), polyethylene glycol diacrylate (ethyleneoxide-added mol number (n)=14) to serve as a polyfunctional monomer,2-hydroxy-2-methyl-1-phenylpropane-1-one (D1173, available from CibaSpecialty Chemicals) to serve as a polymerization initiator were mixedtogether in the proportions shown in Tables 1 and 2 below. Using a rollcoater, the composition was applied to a polyethylene terephthalate filmand was irradiated with a UV ray with a wavelength of 365 nm. The UV raywas shone at an energy density of 2000 mJ/cm² to cause the compositionto polymerize and thereby form a polymer film. The polymer film was thenpeeled off the polyethylene terephthalate film to obtain asingle-layered, liquid absorbing sheet. TABLE 1 Examples (parts byweight) Components 1 2 3 4 5 6 Phenoxypolyethylene 50 50 — — — — glycolacrylate (n = 6) Methoxypolyethylene — — 50 50 — — glycol acrylate (n =3) Methoxypolyethylene — — — — 50 50 glycol acrylate (n = 9)Acryloylmorpholine 50 — 50 — 50 — N,N-diethylacrylamide — 50 — 50 — 50Polyfunctional monomer 0.5 0.5 0.5 0.5 0.5 0.5 Polymerization 0.5 0.50.5 0.5 0.5 0.5 initiator

TABLE 2 Comparative Examples (parts by weight) Components 1 2 3 4 5 6 7Phenoxypolyethylene 100 — — — 50 — glycol acrylate (n = 6)Methoxypolyethylene — 100 — — — 50 — glycol acrylate (n = 3)Methoxypolyethylene — — 100 — — — — glycol acrylate (n = 9)Acryloylmorpholine — — — 100 — — 50 N,N- — — — — 100 — 50diethylacrylamide Polyfunctional 0.5 0.5 0.5 0.5 0.5 0.5 0.5 monomerPolymerization 0.5 0.5 0.5 0.5 0.5 0.5 0.5 initiator

Meanwhile, an electrolyte solution was prepared by adding apredetermined amount of a lithium salt as an electrolyte to a mixedsolvent containing equal volumes of dimethyl carbonate, propylenecarbonate, and ethylene carbonate. 0.2 ml of this electrolyte solutionwas added dropwise to a 0.03 g piece of the liquid absorbing sheetobtained above and the time it took for the liquid absorbing sheet tocompletely absorb the electrolyte solution and swell was visuallydetermined. The liquid absorbing sheet was also immersed in a sufficientamount of the electrolyte solution at 23° C. After 3 hours, theappearance of the liquid-absorbing resin layer was visually observed.The liquid absorbing sheet was pulled out of the mixed solvent and thesolvent remaining on the surface of the sheet was immediately wiped off.The liquid absorbing sheet was then weighed and its swell ratio wasdetermined. Furthermore, the liquid absorbing sheet was heated in a wetheat oven (40° C., 90% RH, 96 hrs) and was examined for the degree ofswelling. The results are shown in Tables 3 and 4 below.

Using hand roller technique (23° C.), a 5 cm wide, 100 μm thickpolypropylene nonwoven fabric was laminated onto each side of the liquidabsorbing sheet obtained above. The adhesion strength was thendetermined on a tensile tester (TENSILON, Orientech) in T-peel mode.Also, a 3 cm wide strip of the liquid absorbing sheet was laminated ontoan Ni surface using hand roller technique (23° C.). The adhesionstrength was then determined on a tensile tester (TENSILON, Orientech)in T-peel mode. The results are shown in Tables 3 and 4 below. TABLE 3Examples Components 1 2 3 4 5 6 (Degree of swelling) W/O wet heatprocess Dropwise 80 90 60 70 30 40 addition (min) Immersed 9.3 9.5 9.09.2 9.4 9.6 (times) (Degree of swelling) With wet heat process Dropwise100 100 70 80 50 70 addition (min) Immersed 9.2 9.3 9.0 9.1 9.2 9.5(times) Adhesion strength to nonwoven fabric Hand roller 0.3 0.2 0.2 0.10.4 0.08 (kg/5 cm) Adhesion strength to Ni surface Hand roller 0.3 0.10.2 0.1 0.4 0.08 (kg/3 cm)

TABLE 4 Comparative Examples Components 1 2 3 4 5 6 7 (Degree ofswelling) W/O wet heat process Dropwise 30 40 15 120<   120<   40 120<  addition (min) Immersed 8.5 8.7 9.0   2.3   2.0 9.1   1.8 (times)(Degree of swelling) With wet heat process Dropwise 40 60 25 120<  120<   60 120<   addition (min) Immersed 8.3 8.4 8.5   2.0   2.0 9.0  1.6 (times) Adhesion strength to nonwoven fabric Hand roller 0 0 0 0 00 0 (kg/5 cm) Adhesion strength to Ni surface Hand roller 0 0 0 0 0 0 0(kg/3 cm)

The results of Examples 1 through 6 shown in Table 3 indicate that thecopolymers formed of one of the polyethylene glycol acrylate monomersand one of the amide bond-containing acrylic monomers each show a highability to absorb the electrolyte solution with or without wet heatprocess and have tackiness to the nonwoven fabric support and the nickelsurface used in battery cases.

On the other hand, the results of Comparative Examples 1 through 5 shownin Table 4 indicate that the polyethylene glycol acrylate monomer aloneor the amide bond-containing acrylic monomer alone does not exhibittackiness. In addition, the results of Comparative Examples 6 and 7prove that the copolymer of two polyethylene glycol acrylate monomers orthe copolymer of two amide bond-containing acrylic monomers does notexhibit tackiness, either.

Example 7 Test for the Ability to Absorb an Electrolyte Solution Using aSimulated Battery Pack

As shown in FIG. 4, a 7.0 cm (l)×7.9 cm (w)×2.3 cm (h) ABS resin box 41was obtained. A 6.5 cm (l)×6.5 cm (w)×100 μm-thick liquid absorbingsheet 42 prepared according to Example 1 was stuck to the bottom of thebox with a commercially available adhesive. Three lithium ion batteries43 were placed on the liquid absorbing sheet 42, and a glass epoxysubstrate 44 as a circuit board was placed adjacent to the batteries.

A hole h was drilled through the side wall of one of the batteries 43that was in the middle of the three. The electrolyte solution leakingfrom the hole was allowed to be absorbed by the liquid absorbing sheet.After drilling of the hole h, the batteries were left for one day andnight and the inside of the battery pack was observed. It turned outthat the glass epoxy substrate was not wet. In addition, the decrease inweight of the battery with the hole h drilled in it was 2.5 g, which wasequal to the increase in weight of the liquid absorbing sheet. Theseobservations suggest that the leaked electrolyte solution was entirelyabsorbed by the liquid absorbing sheet.

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided a liquid absorbingsheet comprising a liquid-absorbing resin layer that can effectivelyabsorb the nonaqueous electrolyte solution used in nonaqueouselectrolyte secondary cells that make nonaqueous electrolyte batterypacks (in particular, lithium ion-based nonaqueous secondary batterypacks). The liquid-absorbing resin layer, which itself shows adhesion,makes it possible to easily attach the liquid-absorbing resin layer to asubstrate or easily attach the liquid absorbing sheet to the batterypack. This attachment is carried out at room temperature and withoutrelying on thermal lamination.

1. A liquid absorbing sheet comprising a liquid-absorbing resin layer,wherein the liquid-absorbing resin layer is obtained by irradiating amonomer composition with an energy ray to polymerize the monomercomposition, the monomer composition containing a monofunctional monomercomponent (A) comprising a polyethylene glycol acrylate monomer and anamide bond-containing acrylic monomer; and a polyfunctional monomercomponent (B).
 2. The liquid absorbing sheet according to claim 1,wherein the polyethylene glycol acrylate monomer is phenoxypolyethyleneglycol acrylate or methoxypolyethylene glycol acrylate.
 3. The liquidabsorbing sheet according to claim 1, wherein the amide bond-containingacrylic monomer is acryloylmorpholine or N,N-diethylacrylamide.
 4. Theliquid absorbing sheet according to claim 1, wherein theliquid-absorbing resin layer is formed over a substrate.
 5. The liquidabsorbing sheet according to claim 4, wherein the substrate is capableof absorbing and retaining a nonaqueous electrolyte solution.
 6. Anonaqueous electrolyte battery pack comprising a nonaqueous electrolytebattery cell, a circuit board, an electrolyte-absorbing element forabsorbing an electrolyte solution in the event of electrolyte leakagefrom the nonaqueous electrolyte battery cell, and a battery caseencasing the battery cell, the circuit board and theelectrolyte-absorbing element, wherein the electrolyte-absorbing elementis formed of the liquid absorbing sheet according to claim
 1. 7. Theliquid absorbing sheet according to claim 2, wherein theliquid-absorbing resin layer is formed over a substrate.
 8. The liquidabsorbing sheet according to claim 3, wherein the liquid-absorbing resinlayer is formed over a substrate.
 9. A nonaqueous electrolyte batterypack comprising a nonaqueous electrolyte battery cell, a circuit board,an electrolyte-absorbing element for absorbing an electrolyte solutionin the event of electrolyte leakage from the nonaqueous electrolytebattery cell, and a battery case encasing the battery cell, the circuitboard and the electrolyte-absorbing element, wherein theelectrolyte-absorbing element is formed of the liquid absorbing sheetaccording to claim
 2. 10. A nonaqueous electrolyte battery packcomprising a nonaqueous electrolyte battery cell, a circuit board, anelectrolyte-absorbing element for absorbing an electrolyte solution inthe event of electrolyte leakage from the nonaqueous electrolyte batterycell, and a battery case encasing the battery cell, the circuit boardand the electrolyte-absorbing element, wherein the electrolyte-absorbingelement is formed of the liquid absorbing sheet according to claim 3.11. A nonaqueous electrolyte battery pack comprising a nonaqueouselectrolyte battery cell, a circuit board, an electrolyte-absorbingelement for absorbing an electrolyte solution in the event ofelectrolyte leakage from the nonaqueous electrolyte battery cell, and abattery case encasing the battery cell, the circuit board and theelectrolyte-absorbing element, wherein the electrolyte-absorbing elementis formed of the liquid absorbing sheet according to claim
 4. 12. Anonaqueous electrolyte battery pack comprising a nonaqueous electrolytebattery cell, a circuit board, an electrolyte-absorbing element forabsorbing an electrolyte solution in the event of electrolyte leakagefrom the nonaqueous electrolyte battery cell, and a battery caseencasing the battery cell, the circuit board and theelectrolyte-absorbing element, wherein the electrolyte-absorbing elementis formed of the liquid absorbing sheet according to claim 5.